A BBC executive was involved in the decision to propose that George Entwistle be awarded a £450,000 payoff – twice the minimum due to him – even though the former director general had lasted only 54 days in the top job at the public broadcaster.

According to BBC sources, Lucy Adams, the BBC's human resources director, helped propose the higher figure to the BBC Trust, which ultimately signed off on the payment, in the words of the chairman, Lord Patten, "to conclude matters quickly".

MPs on the powerful public accounts committee will on Thursday question two BBC executives – although not Adams – and a member of the broadcaster's governing trust over the size of the payment, as part of a wider hearing that will also examine the BBC's practice of paying its stars via personal companies that could be used to avoid tax.

Patten and the BBC Trust have come under fire for the level of the payoff after Entwistle performed so poorly in the top job, struggling with a parliamentary performance and media interviews. But there has been little detail as to who helped put it forward to the trust for approval – with some sources indicating there was some "back and forth" with the BBC's regulator over what to pay.

Adams, an ally of Entwistle, was brought back on the broadcaster's executive board by the former director general of the BBC.

There have also been questions raised about why Egon Zehnder was not forced to refund its near £200,000 fee for running the executive search process, given Entwistle's extremely short time in the role. Broadcast industry sources said it was considered best practice to have a clause in any executive search contract that would require a full or partial refund if the successful candidate failed to remain in place for a minimum period of time – ranging from three months to a year.

However, it is understood that the BBC Trust did not have a refund clause in its contract with Egon Zehnder, whose principal on the project is Dom Loehnis, is also a friend of David Cameron. Instead, the executive search firm is now informally helping in the hunt for a replacement for free. Zehnder charged the BBC £157,000 plus VAT.

The rest of the public accounts committee hearing will be taken up by the inquiry into how top stars are paid, after the BBC said it would change a policy in which it encouraged top stars to set up their own companies to receive six or seven figure earnings.

The finance director, Zarin Patel, who is giving evidence on Thursday morning, said that although the BBC did not itself avoid any tax by paying stars this way, it did "give rise to the public perception that individuals use these to avoid tax".